1. Field of the Invention
The present invention relates to an apparatus for estimating friction coefficient between tire and road surface and more particularly an apparatus for properly estimating friction coefficient between tire and road surface according to vehicle traveling conditions.
2. Discussion of Related Arts
In recent years, numerous vehicle control technologies such as a traction control technology, a braking force control technology, a torque distribution control technology and the like, have been proposed and some of these control technologies have been realized in actual automobile markets. Many of these control technologies use a friction coefficient between tire and road surface (hereinafter, referred to as road friction coefficient) for calculation or correction of control parameters. Accordingly, in order to make the control sure and precise, it is necessary to estimate accurate road friction coefficients. Several technologies in which road friction coefficients are estimated by comparing actual values (values detected by sensors, values estimated by observers) with reference values of parameters obtained from a vehicle motion model, have been proposed. For example, in Japanese Patent Application Unexamined No. Toku-Gan-Hei 10-242030, the applicant of the present invention proposes technologies wherein road friction coefficients are estimated by comparing slip angles estimated by observers with reference values of slip angles based on a vehicle motion model on road surface with high friction coefficients or on road surface with low friction coefficients.
In case where physical quantities such as road friction coefficients which are unable to be measured directly are estimated, generally their responsiveness, accuracy and stability are in a trade-off relationship with each other. That is, if the control system tries to catch changes of road friction coefficients quickly, there is a high possibility that the control system responds to disturbances and sensor noises and as a result road friction coefficients are erroneously estimated. On the other hand, in order to avoid this erroneous estimation, the deviation of road friction coefficients must be deleted by processes such as applying a strong filtering process in the calculation of road friction coefficients.
Specifically, in case where a vehicle enters from a dry paved road into a frozen road during traveling, road friction coefficients abruptly change from a high friction state to a low friction state. In this situation, it is necessary to recognize that the vehicle comes into a road surface having low friction coefficients as soon as possible. However, in case of a small steering angle, or in case of detecting a small yaw rate value or in case of detecting a small lateral acceleration value, sometimes the recognition of low road friction coefficients is delayed. On the other hand, in case where this recognition of low road friction coefficients is too sensitive, small noises make the road friction coefficient estimating apparatus erroneously recognize normal dry roads as roads having low friction coefficients.
To solve this problem, in Japanese Patent Application Laid-open Toku-Gan-Hei 11-267730, the road friction coefficient estimating apparatus includes a road friction coefficient estimating means dedicated to a road surface with low friction coefficient in the road friction coefficient estimating means so as to swiftly be able to establish a road friction coefficient according to vehicle running conditions even when a vehicle travels in the midst of low friction environments.
However, the aforesaid technique requires two road friction coefficients estimating means, one is an ordinal road friction coefficient estimating means and another is a low friction road dedicated friction coefficient estimating means. Accordingly, there is a defect that the control system becomes complicated, large-sized and expensive.
It is an object of the present invention to provide an inexpensive road friction coefficient estimating apparatus for a vehicle capable of efficiently estimating friction coefficients of road surfaces according to road conditions without complicating and up-sizing the apparatus.
To achieve the object, the road friction coefficient estimating apparatus for a vehicle, having an estimating means for estimating a road friction coefficient based on motion parameters indicative of a motion of said vehicle, comprises a road roughness detecting means for detecting a roughness of road surface and for producing a road roughness parameter indicating the roughness of road surface, wherein the estimating means is adapted to vary a responsibility of estimating the road friction coefficient according to the road roughness parameter.